1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine and a printer that employs an electrophotographic method.
2. Description of the Related Art
In recent years, an image forming apparatus which employs an electrophotographic method to form a color image has been widely used. Such a color image forming apparatus is further divided to various methods. For example, there is a method in which a toner image of each color is sequentially superposed on one another and transferred to an intermediate transfer member. The toner images on the intermediate transfer member are then collectively transferred to a transfer material (e.g., a paper sheet or a transparent film). Further, there is a method in which a toner image of each color is sequentially superposed on one another and transferred to a transfer material carried on a transfer material conveyance member. An endless belt is often used as the intermediate transfer member and the transfer material conveyance member (hereinafter referred to as an intermediate transfer belt and a conveyance belt respectively).
An in-line type image forming apparatus transfers toner images from a plurality of image bearing members arranged in a line to the intermediate transfer belt. The in-line type image forming apparatus takes into account a retransfer phenomenon in which a portion of the toner image previously transferred to the intermediate transfer belt is retransferred to the image bearing member in the transfer process. Further, the in-line type image forming apparatus takes into account a difference between transfer efficiencies of toners of each color generated in the transfer process.
When the image forming apparatus forms a secondary color image by superposing two colors, if the difference between the transfer efficiency of each color toner is not appropriately taken into account, it may generate density unevenness. Density unevenness is a phenomenon in which the color of the toner image transferred to the transfer material becomes different from the predetermined color. The density unevenness is caused by the transfer efficiency of the toner image of a second color becoming lower than the transfer efficiency of the toner image of a first color. When the toner image of the first color is transferred to the intermediate transfer belt, the charge amount of negative polarity on the intermediate transfer belt is increased, thus the transfer efficiency of the toner image of the second color is lowered. More specifically, if the charge amount of negative polarity on the intermediate transfer belt is large, an electric field that repels the positive electric field for transferring the second color toner to the belt becomes large. As a result, the transfer efficiency of the second color toner may be lowered. In particular, if the image forming apparatus forms a secondary color image of maximum density, a large amount of the first color toner is transferred to the intermediate transfer belt, so that the charge amount of negative polarity on the belt becomes large. The transfer efficiency of the second color toner then becomes lowered to cause density unevenness.
Japanese Patent Application Laid-Open No. 5-45994 discusses a configuration which transfers a greater amount of the second color toner (i.e. an amount of the toner formed on a photosensitive drum) as compared to the amount of the first color toner. The toner amount of the second color to be transferred to the intermediate transfer belt is increased to compensate for lowering of the transfer efficiency of the second color and thus to reduce occurrence of the density unevenness.
However, the technique discussed in Japanese Patent Application Laid-Open No. 5-45994 does not resolve deterioration itself of the transfer efficiency of the second color and subsequent ones. As a result, a consumed amount of the toner increases according to the technique discussed in Japanese Patent Application Laid-Open No. 5-45994.